c++11 interprocess atomics and mutexes

Question

I have a Linux program which spawns several processes (fork) and communicates through POSIX Shared Memory. I\'d like to have each process allocate an id (0-255). My intentio

Answer 1

C++11 threading primitives (mutexes, atomics, etc) are threading primitives. They know nothing about processes, and they are not a means of achieving inter-process communication.

As the C++11 standard makes no mention of processes or inter-process communication, the behavior of such primitives when placed in process-shared memory (a memory-mapped file, some kind of out-of-process globally mapped memory, etc) is undefined.

Answer 2

You can use mutex inside of shared memory block, but the mutex must be declared as SHARED, therefore is not unusual using mutexes inside of share memory, u can make own class, it is very simple:

class Mutex {
private:
    void *_handle;
public:
    Mutex(void *shmMemMutex,  bool recursive =false, );
    virtual ~Mutex();

    void lock();
    void unlock();
    bool tryLock();
};

Mutex::Mutex(void *shmMemMutex, bool recursive)
{
    _handle = shmMemMutex;
    pthread_mutexattr_t attr;
    ::pthread_mutexattr_init(&attr);
    ::pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
    ::pthread_mutexattr_settype(&attr, recursive ? PTHREAD_MUTEX_RECURSIVE_NP : PTHREAD_MUTEX_FAST_NP);

    if (::pthread_mutex_init((pthread_mutex_t*)_handle, &attr) == -1) {
        ::free(_handle);
        throw ThreadException("Unable to create mutex");
    }
}
Mutex::~Mutex()
{
    ::pthread_mutex_destroy((pthread_mutex_t*)_handle);
}
void Mutex::lock()
{
    if (::pthread_mutex_lock((pthread_mutex_t*)_handle) != 0) {
        throw ThreadException("Unable to lock mutex");
    }
}
void Mutex::unlock()
{
    if (::pthread_mutex_unlock((pthread_mutex_t*)_handle) != 0) {
        throw ThreadException("Unable to unlock mutex");
    }
}
bool Mutex::tryLock()
{
    int tryResult = ::pthread_mutex_trylock((pthread_mutex_t*)_handle);
    if (tryResult != 0) {
        if (EBUSY == tryResult) return false;
        throw ThreadException("Unable to lock mutex");
    }
    return true;
}